In the design, fabrication and use of gas turbines, there has been an increasing tendency toward higher firing temperatures to optimize turbine performance. Also, as existing turbine airfoils reach the end of their life cycle, it is desirable to replace the airfoils, while simultaneously enhancing performance of the turbine through redesign of the airfoils and accommodating the increased firing temperatures. Enhanced cooling capability at higher firing temperatures with consequent extension of the life of the replacement airfoils is therefore highly desirable. For example, the life cycle of the airfoils for early-produced units of the MS6001 B gas turbine, manufactured by assignee, is nearing an end. Hence, a new airfoil capable of operating at increased firing temperatures and compatible with such existing gas turbine but with enhanced cooling and extended life is deemed desirable.
A major failure potential for an airfoil is its margin for creep. With airfoil time at operational temperature and at a given stress level, the airfoil may tend to stretch and to develop a crack or a creep void if not cooled properly. The formation of a crack or creep void reduces surface area, which in turn increases the stress and may cause the blade to rupture or crack. Thus, when redesigning an airfoil for an existing gas turbine, particularly for operation at increased firing temperatures, enhanced cooling and consequent reduction in the bulk temperature of the airfoil is highly desirable to increase the creep margin and airfoil life. Airfoil redesign is also desirable without altering or changing any other part of the turbomachinery and particularly without changing the attachment of the airfoils to the turbine wheel. That is, the desired airfoil redesign is constrained by the original design constraints of existing turbomachinery in which the new airfoil may be employed as a replacement part. Performance is also a significant consideration. For example, boundary layer separation from and reattachment to the airfoil surface may occur. Additionally, shock waves may form on the leading edge of the airfoil. These and other factors contribute to an increase in the temperature of the airfoil, degrade performance and are to be avoided.